1. Field of the Invention
This invention relates to focal plane readout unit cells, and particularly to techniques for sensing induced pulses and diverting them from a unit cell's signal path.
2. Description of the Related Art
A focal plane array is made from an array of photodetectors that are coupled to respective "readout unit cells". The unit cells integrate the photocurrent produced by their respective photodetector over a specific integration period, and the integrated currents are multiplexed and amplified to produce a single video output.
One problem faced by focal plane arrays, particularly infrared arrays operating in the presence of nuclear radiation, for example, is that of ionizing radiation which is absorbed by the photodetectors. The radiation, which can include beta (.beta.) and gamma (.gamma.) rays, deposits a charge packet on the photodetector's p-n junction capacitance. The charge packet causes a voltage step to appear across the junction, inducing extremely short pulses or spikes in the photocurrent output. In a typical application, as many as 4-5 pulses can be induced in the course of a typical integration period (.about.8 ms), which introduce undesirable noise into the unit cell's signal path.
Efforts have been made to reduce or eliminate the negative impact of such induced pulses. One method is referred to as "sub-frame averaging", and is described in U.S. Pat. No. 5,119,321 to Burton, Jr. et al., for example. Many frames of video data are taken within the integration period, and signal processing is used to eliminate the frames that are corrupted by noise pulses. However, detecting corruption due to noise pulses is difficult for the small but non-negligible pulses typically induced by radiation absorption. Also, the elimination of corrupted video data makes it difficult or impossible to preserve the systems' radiometric calibration. Nor can the circuitry required to perform sub-frame averaging be practically realized within the area of a unit cell, arrays of which are typically integrated together on a common substrate.
A need exists for a circuit or method that reduces or eliminates the negative impact of pulses induced in a photodetector's photocurrent output, which can be realized within the area allotted a unit cell.